1. Field of the Invention
The present invention relates to the manufacture of printed circuits. It especially concerns the manufacture of printed circuit substrates, for example multilayer substrates, by silk screen printing deposit of a conducting ink and an insulating ink on an insulating substrate.
Baking is of prime importance in the manufacture of such a substrate. It is in fact during this very high temperature heat treatment that the materials acquire their mechanical and electrical properties, and the polymer resins or other organic constituents which form the temporary binding of the inks are eliminated.
2. Description of the Prior Art
The ovens used at the present time for carrying out this baking operation have been especially designed for baking in an oxidizing atmosphere (air) silk-screen printing conducting inks with a noble metal base such as gold.
These are continuous ovens, with a conveyor belt, having accesses toward the outside for introducing and receiving substrates, gas injection means for carrying out the baking in the desired atmosphere (air for noble metal based silk screen printing inks) and gas extraction means for extracting the residues obtained by elimination of the polymer resins during baking.
The elimination of the polymer resins, which takes place at the beginning of the baking cycle between 250.degree. C. and 500.degree. C., produces reducing gases.
When the baking takes place in air, which is the case for noble metal based silk screen printing inks, these reducing gases combine with the oxygen in the air before being eliminated by the extraction means;
On the other hand, when the silk screen printing ink used is based on a non-noble metal such as copper, baking requires a nitrogen atmosphere (neutral gas) with a very low oxygen concentration (partial oxygen pressure 10.sup.-5) so as to avoid oxidation of the copper due to the reactivity thereof.
Baking in a non-oxidizing atmosphere then raises a fundamental problem, which is the rapid elimination of these reducing gases. In fact, if certain conditions are present (proportion and type of polymer, operating conditions of the gases and loading rate of the oven), the presence of these reducing gases can seriously affect the properties of the materials. The ovens used at the present time do not solve this problem.